Piezoelectric accelerometers of various designs have been used for decades in connection with structureborne and fluidborne sound measurements. A broad set of applications where they have been used include vibration monitoring of machinery, shock evaluation of structures, seismic sensing, and underwater acoustic surveillance. When low frequency applications are considered (e.g., frequencies below 10 kHz) flexural mode accelerometers are often used because they have excellent performance characteristics and can be fabricated in a reasonably straightforward manner. High frequency applications are better served with compression and shear mode accelerometers because the resonance frequency of such devices is typically in the ultrasonic frequency range and therefore facilitates a flat receiving sensitivity over a relatively large bandwidth. For a general discussion on the basic operating principles of accelerometers, refer to G. Gautschi, Piezoelectric Sensorics: Force, Strain, Pressure, Acceleration, and Acoustic Emission Sensors, Materials and Amplifiers (Springer, Berlin, 2006) pp. 167-197, incorporated by reference herein.
Historically speaking, the most pervasive flexural mode accelerometer design is the so-called trilaminar piezoelectric cantilever beam in which a sensing structure comprised of a fixed-free metal beam outfitted with a pair of piezoelectric plates is used to convert dynamic motion to an output voltage that can be processed and displayed to glean useful information about a measurement. Depending on the design, a proof-mass may optionally be included at the free end of the beam so that the operational bandwidth and sensitivity are tuned to specific values. Examples of devices that utilize cantilever beam accelerometers include those described in U.S. Pat. Nos. 2,722,614, 4,333,029, and 4,709,359, each incorporated by reference herein. In all cases it is important to note that the piezoelectric plates associated with these devices comprise a polycrystalline ceramic composition such as lead zirconate titanate (PZT) and the electrical signal is routed from the transducer to the processing electronics/instrumentation using wires that are in intimate electrical contact (e.g., soldered) with the piezoelectric plates.
In the late 1990's, researchers discovered that relaxor-based piezoelectric single crystal materials had superior elasto-piezo-dielectric properties to those of polycrystalline ceramics. Initially, binary formulations comprised of lead magnesium niobate-lead titanate (PMN-PT) and lead zinc niobate-lead titanate (PZN-PT) were developed, but later on ternary compounds comprised of lead magnesium niobate-lead indium niobate-lead titanate (PMN-PIN-PT) and lead magnesium niobate-lead zirconate-lead titanate (PMN-PZ-PT) were developed. Eventually, practical devices containing single crystal transduction elements were made and included a trilaminar cantilever beam accelerometer, such as the one disclosed in U.S. Pat. No. 7,104,140 B2, incorporated by reference herein. Here it is noted that the accelerometer described in this patent contains <011> poled PMN-PT transduction elements and a proof-mass.
All of the research performed to date indicates that one of the main drawbacks of using single crystal materials concerns the undesirable change in the crystal structure and/or depolarization that can occur at moderately elevated temperatures. For example, in binary formulations the rhombohedral-to-tetragonal transition temperature Trt is typically less than 110° C. and the Curie temperature Tc is typically less than 150° C. Moreover, in ternary formulations Trt and Tc moderately exceed 110° C. and 150° C., respectively. So far, none of the research performed to date indicates that conventional soldering techniques can be used in connection with attaching electrical leads to single crystal transduction elements. This is because most solders need to be heated to over 250° C. in order to form satisfactory electrical connections. In contrast, the Curie temperature for most piezoelectric ceramic materials is at least 300° C., therefore conventional and advanced soldering techniques can be employed for ceramic-based transducers. For the case of single crystal transducers, special low temperature solder, conductive epoxy, or novel electroding techniques are required to resolve this issue. These steps are invariably cost-prohibitive and time-consuming.
For a discussion regarding the temperature characteristics associated with binary and ternary single crystal materials, see for example, U.S. Pat. No. 20090194732 A1 and C. H. Sherman and J. L. Butler, Transducers and Arrays for Underwater Sound (Springer, New York, 2007), pp. 552-553, each incorporated by reference herein.